Local temporal variability reflects functional network integration in the human brain: On the crucial role of the thalamus

Local moment-to-moment variability exists at every level of neural organization, but its driving forces remain opaque. Based on animal work showing that within-region variability can be a primary function of synaptic input, we used fMRI to test in humans whether greater temporal variability in local brain regions was associated with higher functional network integration. We found that individuals with higher local temporal variability had a more integrated (lower-dimensional) network fingerprint, with temporal variability in the thalamus showing the strongest association with network dimensionality. Beyond animal findings showing that local variability should upregulate from thalamus to visual cortex, we found that local variability upregulated from thalamus to all structurally connected cortical targets. Critically, greater within-person thalamo-cortical upregulation was associated with higher integration in 12 of 13 examined networks. Our findings indicate that local variability primarily reflects functional integration, with the thalamus acting as a relay or modulator of variability in cortex.